Polyester-type polymers, particularly polyethylene terephthalate (i.e., terephthalic acid-ethylene glycol polycondensation product) generally called a “PET resin” is widely used for providing a packaging material for beverages having a shape of typically a bottle, that is so-called “PET bottle”, because of its transparency, rigidity, easiness of forming, etc.
The scope of applied uses is being enlarged year by year, and some application requires the minimization of oxygen or carbon dioxide transmission therethrough depending on the content materials, so that it is desired to improve the gas-barrier property of PET resin.
As a means for improving the gas-barrier property of PET resin, a multilayer product of PET and polyglycolic acid resin has been disclosed (U.S. Pat. No. 4,424,242 to Barbee), but in the Examples thereof, the films of these resins were pressed together at 210° C., so that because of poor adhesion therebetween, these films are presumed to be liable to peel from each other and be accompanied with difficulty, such as poor appearance and difficulty in maintenance of the performances.
For providing PET bottles, PET is frequently formed by the so-called hot parison scheme wherein PET is stretched and blown within a period prior to crystallization of PET, or the so-called cold parison scheme wherein after being injection-molded, PET is quenched to form an amorphous product (called a preform), which is reheated to above Tg, stretched and blown.
Recently, there has been studied a trial of forming a multilayer product of polyglycolic acid resin and such a PET resin and stretch-blowing it to form a multilayer bottle. However, as polyglycolic acid resin has high crystallinity, it causes whitening due to crystallization before stretch-blowing either according to the hot parison scheme or according to the cold parison scheme. When such a crystallized polyglycolic acid resin is forcibly stretched, it is liable to cause puncture or breakable, or stretching irregularity. For these difficulties, it has been believed impossible at all to form a multilayer stretched product, such as gas-barrier bottle, including polyglycolic acid resin.